Chemically-strengthened glass articles, also known as ion-strengthened glass articles, are used in a variety of applications. Perhaps the best-known example of a chemically-strengthened glass article is the scratch-resistant faceplates used in portable electronic devices, e.g., the faceplates made from Corning Incorporated's Gorilla® Glass. In broad overview, such articles are made by forming a glass having a composition suitable for chemical strengthening into a desired configuration, e.g., into a glass sheet in the case of faceplates, and then subjecting the formed glass to chemical strengthening through an ion exchange process, e.g., a treatment in which the formed glass is submersed in a salt bath at an elevated temperature for a predetermined period of time.
The ion-exchange process causes ions from the salt bath, e.g., potassium ions, to diffuse into the glass while ions from the glass, e.g., sodium ions, diffuse out of the glass. Because of their different ionic radii, this exchange of ions between the glass and the salt bath results in the formation of a compressive layer at the surface of the glass which enhances the glass's mechanical properties, e.g., its surface hardness. The effects of the ion exchange process are typically characterized in terms of two parameters: (1) the depth of layer (DOL) produced by the process and (2) the final maximum surface compressive stress (CS). Values for these parameters are most conveniently determined using optical measurements, and commercial equipment is available for this purpose, e.g., instruments sold by Frontier Semiconductor and Orihara Industrial Company, Ltd. As used herein, DOL and CS values are values determined using such equipment.
The selection of batch components for producing glasses suitable for chemical strengthening has been a challenging process involving producing numerous glass samples having different compositions, subjecting those samples to an ion-exchange process, and then testing the resulting ion-exchanged glasses for their DOL and CS values. As a result of this complexity, there exists a need for a systematic approach for selecting components and/or component concentrations of glasses that are going to under go chemical strengthening.
The present disclosure addresses this need for the specific case of glasses that contain SiO2 and B2O3 as network formers (referred to herein as borosilicate glasses). As discussed below, in accordance with the disclosure, measured and/or estimated values for the coordination state of B2O3 have been found predictive of the chemical-strengthening properties of borosilicate glasses and thus useful in selecting components of such glasses and/or component concentrations. In particular, measured and/or estimated values for the coordination state of B2O3 can be used to improve such chemical-strengthening properties as mutual diffusivity, maximum surface compressive stress, and/or indentation threshold.